There exist two major bottlenecks of technical challenges for economical ethanol production using lignocellulosic biomass as feedstocks. First, inhibitory compounds associated with lignocellulo sic biomass pretreatment, especially by dilute acid hydrolysis, inhibit microbial growth and subsequent fermentation. Major inhibitor include 2-furaldehyde (furfural) and 5-(hydroxymethyl)-2-furaldehyde (HMF) are derived from lignocellulosic hydrolysates. Bacteria and yeast are susceptible and in general unable to grow in the presence of multiple inhibitors even at low concentrations. Another significant technical challenge is to enable and enhance yeast capability in utilization of pentose such as xylose and arabinose harbored in biomass.
Genetic engineering efforts have been made to improve xylose utilization by overexpressing genes encoding pentose phosphate pathway (PPP) enzymes to enhance xylose flux into central carbon metabolism. For native S. cerevisiae, there is no xylose-specific transporters available and the xylose uptake is via certain hexose transporters such as Hxt4, Hxt5, Hxt7. Recently, several heterologous sugar transporter genes possessing xylose transport functions have been expressed in S. cerevisiae such as SUT1, XUT1 or XUT3 from S. stipitis, At5g59250 and At5g17010 from A. thaliana, An25 from N. crassa, DEHA0D02167 and XylHP from D. hansenii, and symporters GXS1 and GXF1 genes from C. intermedia. Improvement of xylose utilization by such efforts was observed but a satisfactory level has not been reached. As such, there is a need to further develop ethanologenic yeast that are tolerance to major inhibitors such as aldehydes and to establish xylose transportation systems to facilitate xylose uptake for efficient lignocellulose-to-ethanol conversion.